1. Field of the Invention
This invention relates to an electrical device, e.g. a transformer or an inductor, which contains a magnetic core, and to a magnetic core half for use in making such a device.
2. Introduction to the Invention
Electrical devices for use as transformers and inductors commonly comprise a magnetic core inserted into a bobbin around which one or more wire coils are wound. When used for power supply, data, or telecommunications applications, such devices are mounted on printed circuit boards (PCBs), along with other electronic components. Due to the large number of components present on the board, it is important that the amount of space occupied by the electrical device be minimized. However, it is also important that the electrical device be appropriately shielded, so as to prevent electrical and magnetic interference with adjacent components.
The conventional approach for reducing the area occupied by the device is to use a smaller conventional part. However, this can give disadvantages in terms of leakage inductance, DC resistance, and total harmonic distortion. Another approach is to mount the wound transformer or inductor in a non-standard orientation, taking advantage of the smallest dimensions available. This approach often requires highly complicated and/or costly mounting fixtures, and the resulting product may be susceptible to radiated emissions, vibration, and/or physical shock. In addition, conventional surface mounting techniques, which use pins or leads extending beyond the edges of the device, increase the space occupied on the board.
A conventional transformer, such as that disclosed in U.S. Pat. No. 4,760,366 (Mitsui), is positioned so that the core, which has a generally flat rectangular shape with curved edges, is parallel to the PCB. It can be mounted onto the board by means of pins. The orientation is such that the height of the device off the board is minimized but the space occupied by the device on the board is at least as large as the largest dimension of the core.
A key factor in determining the footprint occupied by the electrical device is the size and shape of the magnetic core. We have now found that a particular core shape, oriented in a particular way on the PCB, allows a high degree of self-shielding in order to minimize radiated emissions which may affect adjacent components, especially at higher frequencies and drive levels. Therefore, in a first aspect, the invention provides an electrical device for mounting on a circuit board, said device comprising a magnetic core and at least one coil wound around the core, which device comprises
(1) a first core half comprising
(a) a first outer skirt comprising first and second side walls connected by a top wall,
(b) a first back-plane connected to the first outer skirt to form a semi-enclosed housing, said first outer skirt and said first back-plane comprising a first magnetic material, and
(c) a first center core component which
(i) has a primary axis and a secondary axis, the secondary axis having a smaller dimension than the primary axis,
(ii) has a cross-section with a ratio of length of primary axis to length of secondary axis of 1.1:1 to 3.5:1
(iii) extends perpendicularly from the first back-plane so that the primary axis is perpendicular to the circuit board when the device is mounted, and
(iv) comprises a second magnetic material,
(2) a second core half is matable with the first core half, and which comprises
(a) a second outer skirt comprising first and second side walls connected by a top wall,
(b) a second back-plane connected to the second outer skirt to form a semi-enclosed housing, said second outer skirt and said second back-plane comprising the first magnetic material, and
(c) a second center core component which
(i) has a primary axis and a secondary axis, the secondary axis having a smaller dimension than the primary axis,
(ii) has a cross-section with a ratio of length of primary axis to length of secondary axis of 1.1:1 to 3.5:1,
(iii) extends perpendicularly from the second back-plane so that the primary axis is perpendicular to the circuit board when the device is mounted, and
(iv) comprises the second magnetic material,
(3) a bobbin which
(a) comprises an exterior surface and a hollow center section having a cross-section of similar shape to the cross-section of the first and second core components and sized for receiving the first and second core components, and
(b) is attached to first and second flanges, each of which flanges comprises wire mounting posts on its bottom edge; and
(4) at least one coil which is wound around the exterior surface of the bobbin,
the first and second core halves being mated so that the first and second core components are inserted into the center section of the bobbin to form the magnetic coil and the coil is attached to at least one pair of wire mounting posts.
The core halves, themselves, also comprise an aspect of the invention. Therefore, in a second aspect, the invention provides a magnetic core half for use in a power supply, or a data or telecommunications circuit which comprises
(1) a first outer skirt comprising first and second side walls connected by a top wall,
(2) a first back-plane connected to the first outer skirt, said first outer skirt and first back-plane
(a) comprising a first magnetic material, and
(b) forming a semi-enclosed shape having a front opening and a bottom opening, and
(3) a first center core component which
(a) has a primary axis and a secondary axis, the secondary axis having a smaller dimension than the primary axis,
(b) has a cross-section with a ratio of length of primary axis to length of secondary axis of 1.1:1 to 3.5:1,
(c) is attached to the first back-plane perpendicularly so that the primary axis is perpendicular to a circuit board when mounted on a circuit board, and
(d) comprises a second magnetic material.
In a third aspect, the invention provides an assembly which
(A) an electrical device according to the first aspect of the invention, and
(B) a circuit board, said electrical device being electrically connected to said circuit board by means of a wire from the coil wound around the mounting posts.